The present invention is directed to the field of solid state lasers and, more particularly, to a system for controlling the optical path difference in the lasing medium.
Solid state lasers typically comprise a lasing medium having a slab geometry. The lasing medium includes side faces and edge faces. During operation of the laser, the side faces are pumped by a radiation source to generate a laser beam. The pumping process creates a substantial amount of excess heat within the lasing medium. Surface cooling of the side faces and edge faces is conventionally utilized to remove some of this excess heat from the lasing medium, so as to control its temperature. Ideally, the temperature distribution throughout the lasing medium is uniform to eliminate temperature gradients which produce associated thermal strains and index variations. Thermal strains produce thermal dimensional distortions in the lasing medium.
Temperature gradients through the thickness of the lasing medium are averaged out by passing the laser beam along the length of the lasing medium in a zig-zag pattern. Temperature gradients across the width of the lasing medium are controlled by using edge control bars disposed at the edge faces.
Surface cooling does not achieve temperature uniformity within the lasing medium, however, and thermal distortion of the lasing medium causes associated detrimental effects on the laser beam. Consequently, the laser beam quality and average power capabilities of known solid state lasers are limited. The thermal strains and associated thermal distortions cause a thermal optical path difference (OPD) in the lasing medium. The OPD causes different portions of the laser beam to travel at different speeds through the lasing medium relative to other portions. As a result, the laser beam has a non-flat wavefront.
It is important to control the OPD in the lasing medium to maintain a high level of optical quality needed to achieve high performance of the solid state laser system. An increase in the OPD results in reduced performance of the laser by limiting the laser beam average power and diminishing the laser beam quality. The OPD can reach such a high level that the laser is unsuitable for use in certain applications such as phase conjugation or unstable resonator which require precise OPD control.
Thus, there is a need for a method and apparatus for closely controlling the OPD in a lasing medium in a solid state laser system that overcomes the above-described deficiencies of known techniques.